This invention relates to a metrological instrument capable of measuring the profile or form (that is the overall shape) of a surface of a workpiece or component.
Taylor Hobson Limited, a division of Ametek Inc., manufactures a number of surface profile or form measuring metrological instruments which use a measurement probe in the form of a mechanical stylus to determine the profile or form of the surface. In many of these instruments, relative movement is effected between the stylus and the component along a measurement path (traditionally denoted as the “x axis”) and surface changes in the z direction cause an arm carrying the stylus to pivot or rotate about an axis as the stylus follows these surface changes. A measurement gauge, in this case an interferometric gauge, measures the z displacement.
One type of such a metrological instrument manufactured by Taylor Hobson is the Form Talysurf PGI (Registered Trade Mark) series. This series of instruments has a particularly good range-to-resolution ratio and so is capable of making measurements of both form and surface roughness or texture on surfaces having a significant degree of form. This makes the use of the Form Talysurf PGI series of instruments particularly advantageous for aspheric surface measurement.
In measuring an aspheric component with such a metrological instrument, it is usual that a measurement be taken over a measurement path between opposed points on the surface and passing through a point on the surface through which an axis of rotation of the component extends, so that in the case of an aspheric body having a circular base the measurement path represents a projection of the diameter of the circular base onto the aspheric surface. For this reason the measurement path may be referred to as a “diametral path”.
The component is usually positioned so that the rotational axis is nominally aligned to the z-axis of the metrological instrument.
In some circumstances it may not be possible to measure the full diametral path of the component due to physical limitations. These limitations might, for example, be that the component dimensions exceed the measurement envelope of the instrument (for example the diameter is longer than the maximum measurement length), or that at least part of the slope of the component surface is too great. Typically, the maximum gradient in the x-z plane which can be measured by a Form Talysurf PGI metrological instrument is about 30°, beyond which the stresses applied to the stylus arm may become too great and may induce bending. Where the surface to be measured is a recessed or concave surface, then another physical constraint may be the dimensions of the measurement probe.
One type of component for which the latter two issues may arise is a mould for small aspheric lenses, particularly lenses for use in digital cameras and mobile telephones because such lenses may have a base diameter in the region of less than 10 millimetres. The issue of surface gradient may also be a problem when measuring the lenses produced by such moulds.
The gradient problem is also becoming increasingly prevalent in the field of lenses for optical storage devices such as Digital Versatile Disc (DVD) recorders and players. This is because, as the demand for increased data storage capacity becomes greater, the optical resolving power of the light used to burn data onto a DVD must become higher. This is being achieved by decreasing the wavelength of the light source from infra-red to blue light. Systems using blue light may, however, require lenses with a greater degree of asphericity and so at least part of the lens surface may have a gradient greater than 30°.